The field of the disclosure relates generally to choices of propellant for a particular application, and more specifically, to propellant properties as well as to methods and systems for propelling an externally powered vehicle.
Proposals for and tests of solar thermal rocket (STR) and laser thermal rocket (LTR) propulsion have primarily used hydrogen as a propellant as it offers the advantage of high specific impulse. For use as a rocket propellant, hydrogen must be liquified, then maintained at a cryogenic temperature. Because cryogenic refrigeration systems are heavy and consume power, the usual approach to using hydrogen as a STR propellant is to consume the hydrogen before much of it boils away. Use is typically expected within a few days or weeks of launch.
STR missions that require on-orbit lifetimes longer than a few months before using the hydrogen are impractical because the hydrogen boils away. Useful missions that require long lifetimes (and are therefore poor candidates for hydrogen-propelled STR) include satellite maneuvering, orbit capture at the end of an interplanetary trajectory, trans-Earth orbit injection for planetary sample return, and satellite station-keeping or orbit maintenance. Another mission, satellite rescue, does not absolutely require a long orbit lifetime but could benefit from long lifetime by using a rescue vehicle for more than one rescue.
Metal parts exposed to hydrogen must be designed with extra strength, or exotic alloys, which add mass and cost. Hydrogen has very low density: 70 kg/m3 as a liquid at atmospheric pressure. A given mass of hydrogen therefore has a large volume. Volume is costly for launch to space, requiring heavy tanks and adding atmospheric drag. Propellant tanks containing hydrogen must be heavy because of the need for good thermal insulation and the tanks must be oversized to accommodate boil off.